Methodology for bias temperature instability test

ABSTRACT

A method for performing a bias temperature instability test on a device includes performing a first stress on the device. After the first stress, a first measurement is performed to determine a first parameter of the device. After the first measurement, a second stress is performed on the device, wherein only the first parameter is measured between the first stress and the second stress. The method further includes performing a second measurement to determine a second parameter of the device after the second stress. The second parameter is different from the first parameter.

TECHNICAL FIELD

This invention relates generally to integrated circuits, and moreparticularly, to the determination of the behavior of integratedcircuits under bias temperature instability stress.

BACKGROUND

Reliability-related issues of integrated circuits have existed fordecades. In the past, bias temperature instability (BTI, often known fornegative bias temperature instability (NBTI) when related to PMOSdevices) related issues have had a significant impact on designs ofintegrated circuits. To predict the behavior and the reliability ofintegrated circuits, simulation program with integrated circuit emphasis(SPICE) aging models need to be established, which requires BTI tests tobe performed in order to collect data from integrated circuit devices.

The most commonly used parameters in the BTI tests are saturation draincurrent (Idsat) degradation ΔIdsat, linear drain current (Idlin)degradation ΔIdlin, and threshold voltage (Vt) degradation ΔVt, whichare indications as to how devices degrade with high-temperature stressand voltage stress. FIG. 1 illustrates a conventional scheme formeasuring one of the parameters, for example, ΔIdsat. The Y-axisrepresents the BTI degradation, which may be the percentage of the Idsatreduction caused by stresses applied to a device. The X-axis representsthe stress time. After each of the stresses, the parameter of interestis measured. It is noted that at the time the parameter is measured, thedevice recovers partially from the stress (note the drop of the dots),and hence the Idsat degradation ΔIdsat is reduced. Using the methodillustrated in FIG. 1, only one parameter, such as Idsat degradationΔIdsat, can be measured.

To establish the aging model, however, ΔIdsat, ΔIdlin, and ΔVt are allrequired. Therefore, all of these three parameters need to be measured,preferably from a same device since the three parameters are highlycorrelated. FIG. 2 illustrates a second methodology for measuring thethree parameters. After a period of stress time, ΔIdsat is measured,followed by the measurement of other parameters. In the illustratedexample, an I-V sweep is performed, so that ΔIdlin and ΔVt can beextracted. However, it is noted that during the time ΔIdsat, ΔIdlin, andΔVt are measured, the device continuously recovers from the previousstress. Therefore, the value of ΔIdsat actually represents a differentdegradation value of the device than the values of ΔIdlin and ΔVt. Forexample, at the time ΔIdsat is measured, the device may be degraded by 5percent, while when ΔIdlin and ΔVt are measured, the device hasrecovered to a degradation level of only 4 percent. Therefore, themeasured ΔIdsat, ΔIdlin, and ΔVt values cannot reflect the realcorrelation between these parameters. The incorrectly reflectedcorrelation will adversely affect the SPICE aging model established fromthese measurement results.

The results obtained using the method shown in FIG. 2 are shown in FIG.3, which illustrates ΔIdsat and ΔIdlin as functions of stress time. FIG.3 reveals several drawbacks in the conventional BTI test methodology.First, it is found that circles, which are measured ΔIdsat values, aregreater than the squares, which are measured ΔIdlin values. Theseresults conflict with the practical situation, in which the ΔIdsatvalues should always be less than the ΔIdlin values. Second, the slope(n) of ΔIdsat is 0.13, which is significantly different from the slopeof ΔIdlin (0.17). This also conflicts with the practical situation, inwhich the slopes of the ΔIdsat values and the ΔIdlin values should besubstantially the same.

In FIG. 3, the incorrect result that ΔIdsat values are greater than theΔIdlin is a consequence of measuring ΔIdsat values before measuringΔIdlin and ΔVt values. If, however, ΔIdlin and ΔVt values are measuredbefore ΔIdsat, ΔIdsat values will be less than the ΔIdlin values. Thisindicates that the measurement errors have dominated the measurement inthe conventional BTI test methodology.

FIG. 4 illustrates threshold voltage degradation ΔVt as a function ofstress time. It is noticed that the extracted slope is 0.217, which isalso significantly different from the slopes of ΔIdsat and ΔIdlin.Again, this conflicts with the real situation, in which the slope of ΔVtshould be substantially equal to the slopes of ΔIdsat and ΔIdlin. A newmeasurement methodology is thus required to overcome the above-discussedproblems and provide a more accurate modeling.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method forperforming a bias temperature instability test on a device includesperforming a first stress on the device. After the first stress, a firstmeasurement is performed to determine a first parameter of the device.After the first measurement, a second stress is performed on the device,wherein only the first parameter is measured between the first stressand the second stress. A second measurement is performed to determine asecond parameter of the device after the second stress. The secondparameter is different from the first parameter.

In accordance with another aspect of the present invention, a method forperforming a bias temperature instability test includes providing adevice and performing a first stress on the device. After the firststress, a first measurement is performed to determine a first parameterof the device. The first parameter is selected from the group consistingessentially of a saturation current degradation, a linear currentdegradation, and a threshold voltage degradation. The method furtherincludes, after the first measurement, performing a second stress on thedevice. After the second stress, a second measurement is performed todetermine a second parameter of the device. The second parameter isselected from the group and is different from the first parameter.Throughout an entirety of the bias temperature instability test, no morethan one parameter is measured between any two consecutive stresses.

In accordance with yet another aspect of the present invention, a methodfor performing a bias temperature instability test includes providing atransistor; and performing stresses on the transistor. A saturationcurrent degradation of the transistor between a first pair ofconsecutive stresses is measured. A linear current degradation of thetransistor between a second pair of consecutive stresses is measured. Athreshold voltage degradation of the transistor between a third pair ofconsecutive stresses is measured. Between either of the first, thesecond, and the third pairs of consecutive stresses, no more than one ofthe saturation current degradation, the linear current degradation, andthe threshold voltage degradation is measured.

The advantageous features of the present invention include reducedstress recovery time in the stress test, so that the correlation betweenthe tested parameters is preserved.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a conventional bias temperature instability testscheme, wherein only one parameter is measured;

FIG. 2 illustrates another conventional bias temperature instabilitytest scheme, wherein three parameters are measured;

FIGS. 3 and 4 are the results obtained using the test scheme shown inFIG. 2;

FIG. 5 illustrates an embodiment of the present invention, wherein onlyone of a plurality of parameters is tested between any two consecutivestresses;

FIG. 6 illustrates exemplary I-V curves obtained before and afterstresses, which are used for determining threshold voltage degradationΔVt; and

FIGS. 7 and 8 are the results obtained using the test scheme shown inFIG. 5.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

A novel bias temperature instability (BTI) test methodology is provided.In the following discussion, saturation drain current degradation ΔIdsat(also referred to as Idsat degradation hereinafter), linear draincurrent degradation ΔIdlin (also referred to as Idlin degradation), andthreshold voltage degradation ΔVt (also referred to as Vt degradation)are used as examples to explain the concept of the present invention.However, the teaching of the present invention is readily applicable tothe measurement of the degradation of other correlated parameters. Also,transistors are used as exemplary test devices, although the embodimentsof the present invention can also be applied to the measurement of othertypes of devices including, but not limited to NMOSFET, High Voltage(HV) devices, and the like.

FIG. 5 illustrates an exemplary test scheme of the present invention,wherein BTI degradation is illustrated as a function of time. The testdevice is a transistor (not shown) having a gate, a drain, and a source.The Y-axis represents the BTI degradation, which may be representedusing Idsat degradation ΔIdsat in percentage, Idlin degradation ΔIdlinin percentage, and/or threshold voltage degradation ΔVt in percentage.The X-axis represents time. In an exemplary embodiment, the X-axis is inlog scale.

In the beginning (time T0) of a BTI test, a first I-V sweep isperformed. In an embodiment, the first I-V sweep is performed byapplying a fixed drain voltage, and sweeping gate voltage from 0V toVCC, so that the corresponding I-V curve is obtained. An exemplary I-Vcurve is shown in FIG. 6 as line 10. After the first I-V sweep, a firststress is performed, in which the corresponding test device is stressedat an elevated temperature, for example, 125° C. A non-zero gate voltagemay be applied to the gate of the test device during the first stressand subsequent stresses of the BTI test. During the first stress and thesubsequent stresses, the degradation of the test devices continues, asshown by the increase in the Y-value of the dots.

After the first stress, a first measurement is performed to measure afirst parameter, for example, Idsat degradation ΔIdsat, although thefirst parameter may also be Idlin degradation ΔIdlin or Vt degradationΔVt. The first measurement starts at time T1 and ends at time T1′.Throughout the description, the terms “measure” and “measurement” areused to refer to the actions for determining ΔIdsat, ΔIdlin, or ΔVt, andmay refer to measuring these parameters directly, or determining theseparameters by measuring Idsat, Idlin, and Vt, respectively, andcalculating ΔIdsat, ΔIdlin, and ΔVt by calculating the differences ofthese parameters between the currently measured values and the valuesmeasured before the stresses. It is realized that during the test timeΔT1, the degradation recovery occurs, as shown by the dropping of thedots in FIG. 5. To reduce the recovery effect, the test time ΔT1, whichequals to (T1′−T1), is preferably as short as possible. As is known inthe art, the test time ΔT1 is related to the capability of the equipmentused for the test, and may be as short as milliseconds. However, longertime may be needed. Since the degree of the recovery is related to thetest time ΔT1, only one measurement of one parameter is performed beforethe second stress, which is started at time T1′, so that the test timeΔT1 is shortened. In the illustrated example, only the Idsat degradationΔIdsat is measured during the first test time ΔT1.

During a second stress, which is also performed at the elevatedtemperature with a stress voltage applied to the test device, thedegradation of the test device continues. At time T2, the second stressis stopped, and a second measurement of a second parameter is performed.In the preferred embodiment of the present invention, the secondparameter is different from the first parameter. Accordingly, the secondparameter is preferably selected from Idlin degradation ΔIdlin andthreshold voltage degradation ΔVt. In alternative embodiments, the firstparameter may be measured again at time T2.

Preferably, the second test time ΔT2, which equals (T2−T2′) is also asshort as possible. More preferably, the second test time ΔT2 ispreferably substantially equal to the first time ΔT1. Accordingly, evenif there is recovery effect during the second test time ΔT2, the degreeof the recovery is similar to the degree of the recovery during thefirst test time ΔT1. Advantageously, by keeping the degrees of therecovery effect the same between different measurements, the correlationbetween different parameters can be maintained. Again, only onemeasurement of one parameter is performed between the second stress anda subsequent third stress.

After the third stress, a third measurement is performed starting attime T3. In an embodiment, the third measurement is performed on aparameter different from the first and the second parameters. In anexemplary embodiment, the first, the second, and the third parametersare Idsat degradation ΔIdsat, Idlin degradation ΔIdlin, and Vtdegradation ΔVt, respectively. In other embodiments, the thirdmeasurement is performed on one of the first and the second parametersthat have already been measured. Again, during the third test time ΔT3,only one of the parameters of interest is measured. The test time ΔT3 ispreferably short, and more preferably substantially equal to test timesΔT1 and ΔT2.

After the third measurement, more stresses are performed, followed bymore measurements, wherein the above-discussed steps may be repeated.Preferably, throughout the entire BTI test, between any two consecutivestresses, one, and only one, measurement of one of the parameters isperformed. For each of the parameters, the measurements are distributedsubstantially evenly throughout the entire test process. Morepreferably, two consecutive measurements (although separated by astress) are preferably (but not required) performed to differentparameters. The parameters are thus preferably measured in a round robinpattern.

Some of the possible combinations of measurements are listed as follows.It is realized that by applying the above-discussed concept, morecombinations of measurement schemes may be developed. The combinationsinclude, but are not limited to:

-   1.    Stress→ΔIdsat→Stress→ΔIdlin→Stress→ΔVt→Stress→ΔIdsat→Stress→ΔIdlin .    . . .-   2. Stress→ΔVt→Stress→ΔIdlin→Stress→ΔIdsat→Stress→ΔVt→Stress→ΔIdlin .    . . .-   3. Stress→ΔIdsat→Stress→ΔIdlin→Stress→ΔIdsat→Stress→ΔIdlin→Stress .    . . .-   4. Stress→ΔIdsat→Stress→ΔVt→Stress→ΔIdsat→Stress→ΔVt→Stress . . .    and-   5. Stress→ΔIdlin→Stress→ΔVt→Stress→ΔIdlin→Stress→ΔVt→Stress→ΔIdlin .    . . .

After the stress and measurement sequence, at time Tend, the stress andmeasurement steps are stopped. The temperature is returned to roomtemperature, and the stress voltage is disconnected from the testdevice, and hence the recovery occurs. After a period of time that islong enough for the device to stop recovering, for example, 30 minutesafter time Tend (indicated as T(end+30)), a second I-V sweep isperformed, for example, using essentially a same method as the first I-Vsweep performed at time T0. The I-V sweeps performed at time T0 and timeT(end+30) may be used for developing correlation of the parameters. Inalternative embodiments, the first and the second I-V sweeps are notperformed.

In order to ensure that test times ΔT1, ΔT2, ΔT3 are substantially equalto each other, the measurement of all parameters are preferably shortenough. Conventionally, threshold voltage degradation ΔVt may bemeasured by performing an I-V sweep. However, I-V sweeps typically takesignificantly longer time than measuring Idsat degradation ΔIdsat, andhence the recovery effect caused by the I-V sweeps is greater than therecovery effect caused by measuring Idsat degradation ΔIdsat. Anexemplary embodiment for measuring Vt degradation ΔVt is provided below,wherein the measurement may be made in a same period of time asmeasuring Idsat degradation ΔIdsat.

In the sub-threshold range, the I-V curves are substantially linear, asshown by straight lines in FIG. 6, which are obtained from I-V sweeps.Line 10 is the I-V curve obtained before a stress, and line 12 isobtained after the stress. The sub-threshold swing ss may be defined as:ss=(Vg1−Vg2)/[log₁₀(Id(Vg1))−log₁₀(Id(Vg2))]  [Eq. 1]Wherein voltages Vg1 and Vg2 are gate voltages, and currents Id(Vg1) andId(Vg2) are the drain currents corresponding to gate voltages Vg1 andVg2, respectively.

The threshold voltage Vt may be defined as the required gate voltage forachieving a pre-specified drain current at a given drain voltage. In anexemplary embodiment, the threshold voltage Vt is defined as therequired gate voltage for achieving a drain current of 1 μA/μm at adrain voltage of 50 mV, wherein the drain current is the current perunit gate width. In the exemplary embodiment as shown in FIG. 6, therespective test device has a gate length of about 10 μm. Accordingly,the drain current corresponding to the threshold voltage will be 10 μm*1μA/μm, which is equal to about 1E-5 amps. It is found from FIG. 6 thatthe current of 1E-5 amps occurs at a gate voltage of about −0.3 volts(for a PMOS device). Accordingly, the threshold voltage at any givenstress time t may be calculated as:ΔVt(t)=[log₁₀(Id(0)/Id(t))]*ss  [Eq. 2]Wherein ΔVt(t) is the threshold voltage degradation ΔVt at time t, Id(0)is the drain current measured before stress, and Id(t) is the draincurrent measured at stress time t. Therefore, by using Equations 1 and2, the threshold voltage degradation ΔVt may be obtained by measuringthe drain current at time t with the gate voltage equal to −0.3V.Accordingly, the threshold voltage degradation ΔVt can be measuredwithout resorting to I-V sweeps.

FIG. 7 illustrates the results obtained from the embodiments of thepresent invention, wherein Idsat degradation ΔIdsat and Idlindegradation ΔIdlin are shown as functions of stress time. Solid pointsindicate ΔIdsat results, while hollow points indicate ΔIdlin results.The corresponding gate voltages for obtaining these results are alsomarked in FIG. 7. Several facts may be observed from FIG. 7. First, foreach of the gate voltages, the corresponding ΔIdlin fit-line is alwayshigher than the ΔIdsat fit-line. This observation is in-line with thepractical situations in which the ΔIdlin values should always be greaterthan the ΔIdsat values. Second, regardless of the values of the gatevoltage, the slopes (n) of ΔIdsat values and ΔIdlin values are veryclose, ranging from about 0.120 to about 0.130.

FIG. 8 illustrates the threshold voltage degradation ΔVt as a functionof stress time. It is noted that the slope (n) is about 0.146, which ismuch closer to the slopes of ΔIdsat and ΔIdlin than the slopes measuredusing conventional schemes.

The embodiments of the present invention have several advantageousfeatures. First, by measuring only one parameter at a time, and henceshortening the time of measurement, the recovery effect is significantlyreduced, and the measured degradation results are more accurate. Second,with the degrees of recoveries being kept the same for differentparameters, the correlation between the different parameters isreflected more accurately by the measurement results. A more accurateSPICE aging model can thus be established.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A method for performing a bias temperature instability test, themethod comprising: providing a device; performing a first stress on thedevice; after the first stress, performing a first measurement todetermine a first parameter of the device; after the first measurement,performing a second stress on the device, wherein only the firstparameter is measured between the first stress and the second stress;and after the second stress, performing a second measurement todetermine a second parameter of the device, wherein the second parameteris different from the first parameter.
 2. The method of claim 1 furthercomprising: after the second measurement, performing a third stress onthe device, wherein only the second parameter is measured between thesecond stress and the third stress.
 3. The method of claim 2 furthercomprising: after the third stress, performing a third measurement onthe device, wherein the third measurement is performed to determine aparameter selected from the group consisting essentially of the firstparameter and a third parameter different from the first parameter andthe second parameter.
 4. The method of claim 3, wherein the thirdmeasurement is performed to determine the third parameter, and whereinthe method further comprises, after the third measurement, performing afourth stress, wherein only the third parameter is measured between thethird stress and the fourth stress.
 5. The method of claim 1, whereinthe first measurement and the second measurement take substantially asame period of time.
 6. The method of claim 1 further comprisingrepeating steps from the step of performing the first stress to the stepof performing the second measurement.
 7. The method of claim 1, whereinthroughout an entire period of the bias temperature instability test, nomore than one parameter is measured between any two consecutivestresses.
 8. The method of claim 1 further comprising: before the stepof performing the first stress, performing a first I-V sweep; after alast stress, waiting a period of time with no stress being performedduring the period of time; and performing a second I-V sweep.
 9. Themethod of claim 1, wherein the first parameter and the second parameterare selected from the group consisting essentially of a saturationcurrent degradation, a linear current degradation, and a thresholdvoltage degradation.
 10. A method for performing a bias temperatureinstability test, the method comprising: providing a device; performinga first stress on the device; after the first stress, performing a firstmeasurement to determine a first parameter of the device, wherein thefirst parameter is selected from the group consisting essentially of asaturation current degradation, a linear current degradation, and athreshold voltage degradation; after the first measurement, performing asecond stress on the device; and after the second stress, performing asecond measurement to determine a second parameter of the device,wherein the second parameter is selected from the group and is differentfrom the first parameter, and wherein throughout an entirety of the biastemperature instability test, no more than one parameter is measuredbetween any two consecutive stresses.
 11. The method of claim 10 furthercomprising: immediately after the second measurement, performing a thirdstress on the device; and immediately after the third stress, performinga third measurement on the device, wherein the third measurement isperformed to determine the first parameter.
 12. The method of claim 10further comprising: immediately after the second measurement, performinga third stress on the device; and immediately after the third stress,performing a third measurement on the device, wherein the thirdmeasurement is performed to determine a third parameter different fromthe first parameter and the second parameter, and wherein the thirdparameter is selected from the group.
 13. The method of claim 12,wherein the first, the second, and the third measurements takesubstantially a same length of time.
 14. The method of claim 10 furthercomprising: before the step of performing the first stress, performing afirst I-V sweep; after a last stress, waiting a period of time with nostress being performed during the period of time; and performing asecond I-V sweep.
 15. A method for performing a bias temperatureinstability test, the method comprising: providing a transistor;performing stresses on the transistor; measuring a saturation currentdegradation of the transistor between a first pair of consecutivestresses; measuring a linear current degradation of the transistorbetween a second pair of consecutive stresses; and measuring a thresholdvoltage degradation of the transistor between a third pair ofconsecutive stresses, wherein between either of the first, the second,and the third pairs of consecutive stresses, no more than one of thesaturation current degradation, the linear current degradation, and thethreshold voltage degradation is measured.
 16. The method of claim 15,wherein the step of measuring the saturation current degradationcomprises: measuring a saturation current; and determining thesaturation current degradation from the saturation current.
 17. Themethod of claim 15, wherein a first interval between the first pair ofconsecutive stresses, a second interval between the second pair ofconsecutive stresses, and a third interval between the third pair ofconsecutive stresses are substantially equal to each other.
 18. Themethod of claim 15 further comprising repeating the step of measuringthe saturation current degradation, the step of measuring the linearcurrent degradation, and the step of measuring the threshold voltagedegradation in a round robin pattern.
 19. The method of claim 15 furthercomprising: performing a first I-V sweep to the transistor; starting afirst one of the stresses; waiting a period of grace time, whereinduring the period of grace time, no stress and no measurement isperformed; and performing a second I-V sweep to the transistor.
 20. Themethod of claim 19, wherein during an entire period between the firstand the second I-V sweeps, no more than one of the saturation currentdegradation, the linear current degradation, and the threshold voltagedegradation is measured between any two consecutive ones of thestresses.